Boffins hide messages in Skype ‘silence packets’

Careful, someone might hear you

A Warsaw-based security researcher says the packets that Skype sends during silence can be used to carry secret messages in a conversation.

When participants in a call are speaking, Skype sends the audio in 130-byte packets, while during silence it sends 70-byte packets. According to New Scientist, Wojciech Mazurczyk of Warsaw University of Technology’s Institute of Telecommunications has created software which he’s dubbed “SkypeHide” to put private, encrypted messages in the silence packets.

This “packet hijack” is hard to detect, Mazurczyk says. “The secret data is indistinguishable from silence-period traffic, so detection of SkypeHide is very difficult,” he claims.

The software was created in collaboration with two other researchers, Maciej Karaś and Krzysztof Szczypiorski.

Mazurczyk says the steganography technique can carry any kind of data – voice, text or video – and while the secret messages only had a data rate of 1K bps during calls, he believes they would be difficult to intercept.

Last year, the now-Microsoft subsidiary denied that it was rewriting its software to be more law-enforcement-friendly. However, agencies worldwide are looking for ways to bring the popular VoIP system under the loving embrace of interception regimes.

The group intends to present SkypeHide at the First ACM Information Hiding and Multimedia Security Workshop, being held at the University of Montpellier in June. ®